1. Field of the Invention
The present invention relates to a control circuit for an electrical power supply system and, more particularly, to a system for supplying control signals to fluorescent lamp load using the power line as the signal carrier.
2. Description of the Prior Art
In the prior art, load control such as dimming fluorescent lamps has normally been accomplished by some control circuit which chops the power supply wave to reduce the total amount of energy delivered to the lamp. The prior self-contained wall mounted dimmers and controllers have employed phase control for dimming which results in reduced power capability. Phase control is often accomplished using a triac which dissipates very large amounts of power at high currents, which requires the dimmers to be derated to typically 600 to 1,000 watts. This limits the number of fluorescent lamp ballasts which can be controlled by a single switch to that number making up the maximum wattage rating of the controller.
3. Summary of the Invention
An object of the present invention is to provide a load controller which utilizes the power line as the communication signal carrier. A more specific object of the present invention is to provide a load control system in which a single controller is capable of controlling all loads on the power line connected to a particular branch without dissipating significant power in the controller. A still more specific object of the present invention is to provide a system employing a frequency modulated signal superimposed upon the power signal to provide a reliable dimming control signal for fluorescent lamp ballasts. Yet another object of the present invention is to provide a redundant signalling controller which continuously transmits control signals on the power line so that the effect of noise upon the control transmission is suppressed.
Accordingly, the present invention includes a plurality of input switches for selecting a desired load level, an electronic gate array for converting the switch positions to a binary control number dependent upon the combination of switch closures detected by the gate array, an integrated circuit device for converting the output of the gate array from a parallel multi-bit data word to a serial multi-bit data word, transmitting means for providing a high-frequency signal to the power line having a frequency dependent upon the data word, and a filter circuit for blocking transmission of the high frequency signal upstream onto the power supply system. In a particular preferred embodiment to the present invention a master on/off switch is connected to a normally on relay to provide a failsafe circuit arrangement, so that in the event of controller failure all power is maintained on at the previously chosen level and is turned on to maximum output on turn on after a controller failure.